1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display characterized by its low profile and light-weight has been extensively used in electric products including cellular phones and personal computers, and the demand thereof has been increasing. However, since a liquid crystal of the liquid crystal display (hereinafter, referred to as “LCD”) does not emit light by itself, a separate illuminating means for illuminating the LCD is needed besides the LCD when used in dark places where sunlight or room light is not satisfactorily available.
The illuminating means used for the cellular phones and personal computers (especially, notebook computers) is required to be low in profile and small in power consumption. To meet the above demand, a sheet-like spread illuminating apparatus of side light type is often applied as an illuminating means. FIG. 12 shows an example of such a spread illuminating apparatus of a side light system.
A spread illuminating apparatus 1 shown in FIG. 12 is an embodiment of a conventional spread illuminating apparatus for what is called “a back-light system”, and is disposed with its front surface 7 facing a back surface (upper side in FIG. 12) of a light-transmissible type LCD 2 opposite to a display surface F (lower side in FIG. 12). Here, the LCD may alternatively be semi-light transmissible.
The spread illuminating apparatus 1 comprises a substantially rectangular light conductive plate (guide-plate) 3 made of a light-transmissible material, a light conductive bar (guide-rod) 4 made of a light-transmissible material and shaped rectangular in cross section, and a pair of spot-like light sources 5 and 5 using light emitting diodes (LEDs).
The light-transmissible type LCD 2 is disposed in such a manner as to face the front surface 7 of the light conductive plate 3 opposite to a back surface 6 (upper side in FIG. 12). And, usually a reflector plate (not shown) is placed on the back surface 6.
The light conductive bar 4 is disposed such with its one side surface 8 located close to or in contact with one end surface 9 of the light conductive plate 3, and the spot-like light sources 5 and 5 are disposed respectively at both ends faces 10 and 10 of the light conductive bar 4. An optical path conversion means 12 is formed on the other side surface 11 of the light conductive bar 4 opposite to the one side surface 8 in order to allow light rays emitted from the spot-like light sources 5 and 5 to enter the light conductive plate 3 through the one end surface 9. The optical path conversion means 12 comprises a plurality of grooves 13 substantially V-shaped in cross section and a plurality of flat portions 14 formed therebetween.
The light conductive plate 3 has a light reflection pattern 15 on its back surface 6 so as to uniformly reflect the light rays guided from the light conductive bar 4 into the light conductive plate 3 through the surface 8 located close to or in contact with the one end surface 9 of the light conductive plate 3 toward its front surface 7.
The light reflection pattern 15 comprises a plurality of grooves 16 substantially V-shaped in cross section extending in parallel with the one end surface 9 of the light conductive plate 3 and a plurality of flat portions 17 formed therebetween.
With the structure above described, the light rays emitted from the spot-like light sources 5 and 5 enter the light conductive bar 4, have their paths changed by the optical path conversion means 12 formed on the side surface 11 of the light conductive bar 4 and enter the light conductive plate 3 through the one end surface 9.
The light rays introduced into the light conductive plate 3 are repeatedly reflected and refracted by the light reflection pattern 15, and proceed toward an end surface thereof opposite to the end surface 9 during which time-most of the light rays go out of the light conductive plate 3 through the front surface 7. Thus, the light-transmissible type LCD 2 set close to the front surface 7 of the light conductive plate 3 can be illuminated so that the light-transmissible type LCD 2 performs luminescent display (indirect luminescence).
However, with the above structure, a certain amount of the light emitted from the spot-like light sources 5 and 5 is consumed by the light conductive bar 4, therefore decreasing that amount of the light to illuminate the light conductive plate 3. As a countermeasure for the above disadvantage, a spread illuminating apparatus has been disclosed in Japanese Patent Publication No. 2001-35222.
In the spread illuminating apparatus disclosed in Japanese Patent Publication No. 2001-35222, as shown in FIG. 13, a light entrance surface 21 is formed at one corner 18 of the light conductive plate 3 in such a manner as to make a right angle with respect to a line (hereinafter referred to as “diagonal line”) 20 connecting the one corner 18 and another corner 19 opposite to the corner 18, the spot-like light source 5 using a light emitting diode is arranged close to or in contact with the light entrance surface 21, a light reflection pattern 15A, which comprises a plurality of arced grooves 16A disposed concentric with respect to the spot-like light source 5 and arrayed at a regular interval, and a plurality of flat portions 17A formed between adjacent grooves 16A is formed on a top surface of the light conductive plate 3, and the light conductive bar 4 employed in the apparatus shown in FIG. 12 is eliminated. Consequently, the light rays emitted from the spot-like light source 5 directly enter the light conductive plate 3, thus cutting the light loss due to the presence of the light conductive bar 4.
In addition, the present inventors have disclosed, in Japanese Patent Application No. 2001-350161, a spread illuminating apparatus, in which each of the grooves 16A has its depth increasing from the diagonal line in order to improve the luminance distribution of the light reflection pattern 15A in a circumferential direction.
However, in the apparatus shown in FIG. 13, depending on the viewing direction, the field of view is restricted. The luminance distribution is measured by a luminance meter 70 set over the light conductive plate 3 in “direction A” tangential to each of the grooves 16A, and in “direction B” orthogonal to each of the grooves 16A as shown in FIGS. 14 and 15, and the resultant luminance distribution characteristics are different as shown in FIGS. 16 and 17. In the direction A, only a slight shift of an angle Φ from a datum 71 (defined as having an angle Φ of 0 degrees) causes the luminance to decrease sharply as shown in FIG. 16. In other words, when viewing angle is only slightly changed from the datum 71, darkness emerges instantly, thus resulting in a narrow field of view in the direction A. This is caused as follows. When the light conductive plate 3 (or the light reflection pattern 15A) is viewed from above, the light rays emitted from the spot-like light source enter the light conductive plate 3 and are incident vertically on the grooves 16A of the light reflection pattern 15A, and consequently are reflected by the grooves 16A and exit out the light conductive plate 3 without slanting in the direction A, therefore going out vertically.